how to test the gap of the vacuume circuit breaker?

Contact gap or wipe measurements are normally detailed in the operating / maintenance manuals. I suggest you sourced the required manual for the make, model number it should be there.

Some manual have complete bottle replacement procedure which requires proper measurement from operating stroke to contact gap measurement.

Procedures are very specific for each make and model.

There is no direct method possible . Pl refer manufacturer's manual. This may be included as factory acceptance test for VCB.

If you look at certain European brands, they provide a red mark on the moving contact of bellows. This is a measure of contact erosion and indirect indication of gap increase .